The increase of CO2 in the atmosphere is suspected to have a major effect on global climate. It is therefore desired that the emission of anthropogenic CO2 into the atmosphere is reduced. In addition to the development of low CO2 emission power plants, energy-saving automobiles and the increased use of renewable energy sources, the permanent storage of CO2 in subterranean geological formations can be an important means for reducing net CO2 emission.
An extensive review of existing CO2 Capture and Storage (CCS) projects and technology is given in the IPCC Special report on Carbon Dioxide Capture and Storage (Carbon Dioxide Capture and Storage, IPCC, 2005, editors: Metz et al., Cambridge University Press, UK).
CO2 injection into a subterranean geological formation for Enhanced Oil Recovery (EOR) has been applied in the Rangely EOR Project in Colorado, USA. A sandstone oil reservoir has been flooded with CO2 by a water-alternating-gas (WAG) process since 1986. In this project, CO2 in a supercritical state is used to extract additional amounts of oil from the otherwise exhausted oil fields in a tertiary oil recovery process. By the end of 2003, 248 active injectors of which 160 are used for CO2 injection and 348 active producers were in use in the Rangely field. Injection of CO2 occurs through openings at various vertical positions over the entire height of the reservoir. Production of oil also occurs at various vertical positions over the entire height of the reservoir. Injection wells and production wells are horizontally offset from each other. The Rangely Project does not store CO2 in aquifers.
The Sleipner Project, operated by Statoil in the North Sea, is a commercial scale project for the storage of CO2 in a subterranean aquifer. CO2 is stored in supercritical state 250 km off the Norwegian coast. About one million tons of CO2 is removed from produced natural gas and subsequently injected underground, annually. CO2 injection started in October 1996 and by 2008, more than ten million tons of CO2 had been injected at a rate of approximately 2700 tons per day. The formation into which the CO2 is injected is a brine-saturated unconsolidated sandstone about 800-1000 m below the sea floor. A shallow long-reach well is used to take the CO2 2.4 km away from the producing wells and platform area. The injection site is placed beneath a local dome of the top Utsira formation. No concurrent removal of brine from the reservoir occurs.
The In Salah CCS Project is an onshore project for the production of natural gas from a gas reservoir located in a subterranean aquifer. The aquifer is located in the Sahara desert. The reservoir is in a carboniferous sandstone formation, 2000 m deep; it is only 20 m thick, and of low permeability. Natural gas containing up to 10% of CO2 is produced. CO2 is separated, and subsequently re-injected into the water-filled parts of the reservoir. No brine solution is removed from the aquifer.
EP-A-1571105 describes methods for CO2 storage, in which CO2 is added at into a water stream, which stream is then pumped into a subterranean geological formation. The method uses chemical reactions with a mineral-forming agent (sulfate/base). The agent is added, unless use is made of particular geological structures, is in particular those structures that by nature contain these agents in large quantities. This method requires complex and expensive technical equipment. Since no concurrent removal of brine from the reservoir occurs, high local pressure at the site of injection may lead to fractures in the sealing geological formations. This increases the likelihood of CO2 escaping from the reservoir into the atmosphere.
JP06170215 describes a method of introducing a mixture of water and CO2 into a subterranean geological formation. For this purpose, the CO2 is mixed with water above the ground, and thereafter the mixture is introduced into the ground, under high pressure. The method requires a supply of liquid CO2, a booster pump, a heat exchanger and a pump to obtain the required pressure. This renders the process energy-intensive and expensive. Injection of the CO2/water mixture into a reservoir increases the reservoir pressure and may lead to fractures in the sealing formations. This increases the risk of CO2 escaping into the atmosphere.
WO 2010/024949 describes methods for sequestering CO2 in an aquifer.
In view of the above described state of the art it is an object of the present invention to provide alternative methods for the permanent storage of CO2 in subterranean geological formations.
It is a further object of the present invention to provide methods which allow for a more efficient use of the storage capacity of geological formations, such as aquifers, for permanent storage of CO2.
It is a further object of the present invention to provide for a safer and more permanent storage of CO2.